Method and apparatus for determining parameters of gaseous substances

ABSTRACT

A method for determining parameters, especially pressure, temperature, concentration, number of particles and particle size distribution, of gaseous substances present in combustion processes and other high temperature processes, comprises transmitting spectrally broad-band light through an object (2) of measurement, spectrally dividing the light transmitted through said object, and recording the spectral distribution of the light in the studied wavelength range a large number of times. Each recording occurs sequentially in that the spectrally divided light is swept relative to a one-channel detector and for such a short time that the total light intensity of the entire wavelength range is constant during each recording. After that, the mean value of said recorded spectral distributions is generated, and the required parameters are calculated on the basis of said mean value spectral distribution, the appearance of said means value spectral distribution, as well as spectra calculated or recorded for known conditions, being utilized for said calculation. 
     An apparatus comprises means for carrying out the method.

The present invention relates to a method and an apparatus fordetermining parameters, especially pressure, temperature, concentration,number of particles and particle size distribution, of gaseoussubstances present in combustion processes and other high temperatureprocesses, in which method light transmitted through or reflected oremitted by the gases is spectrally divided, and the spectraldistribution of the light in the studied wavelength range is recorded alarge number of times.

The rise in raw material prices and the increasing insistence onefficient measures against pollution have intensified the interest in,for example, combustion control. An American study from the middle ofthe 1970's shows for example that, if the efficiency of combustionprocesses could be increased by 1%, this would mean a saving of 15million barrels of oil per annum in the USA alone. The combustion incoal-burning power plants is another example of a combustion processwhere an efficient control could save large sums of money. Thus, if thetemperature in a coal-burning power plant becomes too high, slagproducts are deposited which, if it comes to the worst, may necessitateclosing down the entire plant for cleaning. Furthermore, the emission ofsubstances hostile to the environment could be reduced if somecombustion processes could be controlled more efficiently, for examplethe incineration of refuse. If the combustion temperature becomes toohigh (above 1600° C.) large amounts of NO are generated, which is one ofthe substances considered to contribute to the so-called forest kill.If, on the other hand, the temperature is too low, dioxine is formedwhich, is a dreaded environmental poison. Suitable control measures tomaintain the temperature at an intermediate value can minimise theemission of these two dangerous substances.

However, to be able to conveniently control the above-mentioned andother processes, transducers are required by which the controlparameters, such as the temperature, the concentration of specificsubstances etc., can be determined.

Unfortunately, the technical progress in this field has not kept pacewith the rise in raw material prices and the environmental issues. Onereason is, of course, the high temperatures which render the use ofconventional transducers and measuring instruments impossible. Anotherreason is that the measuring environment in, for example, a coal-burningpower plant places high demands on the measuring equipment which must beunsusceptible to dirt, vibrations etc. A third reason is the turbulentcharacter of the measuring environment, i.e. the measuring conditionsare subject to strong variations in time, which makes it difficult todesign a measuring equipment capable of operating with great accuracyand reliabilty, and up to a few years ago only unreliable methods wereavailable for determining temperature, concentration and otherparameters. For example, the temperature was measured by means ofthermocouples which interfered with the processes and therefore did notgive reliable results. Furthermore, the concentration of the substancespresent during combustion was recorded by exhausting gas from thecombustion zone and injecting it into, for example, a mass spectrometer.Also these concentration measurements were unreliable because exhaustingof the sample interfered with the processes, the exhausted gas samplewas cooled in the mass spectrometer, and there was a risk that thesubstances could interreact in the mass spectrometer so that thesubstances measured were different from those which were present duringthe process.

When it is desired, in other technical applications, to prevent themeasurement from interfering with the process, use is frequently made ofoptical measuring techniques. One group of optical measuring techniquesis based on the principle of developing by means of light a secondaryeffect, for example fluorescence, which carries information about therequired parameter and the intensity of which is measured. Examples ofsuch techniques are Raman spectroscopy, laser induced fluorescence etc.However, these techniques are not readily applicable to full-scalecombustion plants because the secondary effect is drowned in the lightfrom the flame.

The only realistic possibility of using optical measuring techniques incombustion plants is to make absorption measurements, i.e. to measurehow much light is absorbed by the flame. Also this technique isdifficult because the particle density in these environments is veryhigh and the light transmission therefore very low. In a normalcombustion plant boiler, the transmission is less than 1%, for whichreason a very strong light source is required in order to provide auseful measuring signal.

Recently, an optical technique has been developed by which non-contactmeasurement of concentration and temperature in combustion processes canbe carried out. This technique which is termed CARS (CoherentAnti-Stokes Raman Scattering) is presented in "Elteknik med aktuellelektronik", 1985:4, pp. 76-80. For CARS measurements, use is made oftwo lasers, one of which is tunable and the other has a fixed frequency.The beams from these two lasers are focused and adjusted such that theyintersect at a specific angle. The area upon which the two laser beamsare to be focused has a surface of some μm². If the focusing does notsucceed, the technique does not work. Furthermore, the frequencydifference between the beams must exactly correspond to the differencebetween two internal energy levels in the molecule one wishes toexamine. As will be immediately apparent from this brief description,the measuring device here concerned is technically highly complicatedand can be installed and operated by specially trained technicians only.Naturally, such a device is highly expensive; the paper mentions the sumof 2 million Swedish crowns for each system, and since these arehigh-power lasers, this sum is not expected to become much lower.

Unfortunately, laser technique also has other shortcomings. Firstly, itmay prove difficult to produce a sufficiently strong measuring signal ifmeasurement is carried out in processes having a high particle contentbecause also the CARS technique utilises secondary light for detection.Secondly, the laser is a noisy light source, and this means that themeasuring accuracy in many cases will not be very good. Thirdly,different measuring setups (lasers) must be used for measurement indifferent frequency ranges. Fourthly, safety problems arise since lightreflected from surfaces reflecting as little as a few percent may causeirreparable damage to the eye.

For measurements in combustion processes and other high temperatureprocesses there thus is a need for a simpler, more reliable and lessexpensive technique which can be utilised by anyone without expertknowledge but which nevertheless stands up to the severe conditionsencountered in an environment with high temperatures and a high degreeof pollution.

A cheaper and less complicated concentration measuring technique whichis used in, for example, chemistry and biology, is the absorptionspectroscopy. This technique involves irradiating a cuvette containing aliquid measuring sample with white light, and recording sequentially, bymeans of a slow-scanning spectrometer, the intensity of the differentwavelengths in the spectrum. A typical scan lasts a few minutes, butthis is no problem because the measuring environment is not turbulent.

A device utilising this technique and capable of measuring in turbulentenvironments is disclosed in European patent application No. 84302093.4.This device comprises a pulsed light source for transmitting lightpulses towards the sample whose concentration is to be measured, aspectrometer for dividing each light pulse transmitted through thesample into at least two predetermined spectral components, a diodearray for generating output signals corresponding to the light intensityof each of said predetermined spectral components, and a mean valuegenerator for calculation for all light pulses of the mean value of thedifference in the intensity of the transmitted light for the said twospectral components.

In this device, the influence of turbulence thus is eliminated by makinguse of a pulsed light source, but this brings the disadvantage that itis difficult to compare spectra taken during different pulses becausethe spectral distribution of the light from the light source varies fromone pulse to another.

A further disadvantage of this device is that it is difficult to recorda spectrum with an acceptable signal-to-noise ratio. One reason for thisis the manner in which the spectrum is recorded, i.e. by means of adiode array. In principle, a diode array comprises a large number ofjuxtaposed detector elements. These detector elements suffer from thedisadvantage that, because of their design, they have a limited lightsensitivity and, furthermore, are mutually different in respect of darkcurrent, amplification, temperature-dependent drift, and drift due toaging. As a result of these factors, the above-mentioned device isunsuitable for the determination of hightemperature process controlparameters.

It therefore is the object of the present invention to provide aninexpensive and simple method for high-precision measurement ofparameters of gaseous substances occurring in combustion processes andother high-temperature processes. An especially interesting applicationis measuring inside flames. Another object of this invention is toprovide an apparatus for carrying the method into effect.

The object of the invention is achieved by means of a method which ischaracterised in that each recording occurs sequentially in that thespectrally divided light is swept relative to a one-channel detector andfor such a short time that the total light intensity of the entirewavelength range is constant during each recording; that the means valueof said recorded spectral distributions is generated; and that therequired parameters are calculated on the basis of said mean valuespectral distribution, the appearance of said mean value spectraldistribution, as well as spectra calculated or recorded for knownconditions, being utilised for said calculation.

To carry the method according to the invention into effect, use is madeof an apparatus which is characterised by means for sequential recordingof the spectral distribution of the light in the studied wavelengthrange in such a short time that the total light intensity of the entirewavelength range is constant during each recording, said meanscomprising a one-channel detector whose output signal is proportional tothe intensity of the received light, and means for sweeping thespectrally divided light relative to said one-channel detector; a meanvalue generator; and computer means for calculating the requiredparameters.

The main advantage of the method and the apparatus according to theinvention is that the spectrum is recorded by means of a one-channeldetector, whereby the measuring errors associated with the differentcharacteristics of the detector elements in a diode array areeliminated. A further advantage is that the detector element employedmay be a photomultiplier having a higher light sensitivity than thedetector elements of the diode array.

If measuring is effected on transmitted or reflected light, theapparatus further comprises a spectrally broad-band continuous lightsource transmitting light towards the object of measurement. Theadvantage of a continuous light source resides in that it can be madeimmensely stable, and that it is more reliable than, for example, apulsed lamp.

Otherwise, the object of measurement itself is used as a light source,and then the emission spectra are investigated.

The present invention makes it possible to carry out measurement on allgaseous substances through which light can be transmitted.

Furthermore, the parameters of several substances may be determined inone and the same measurement, which is of course an advantage.

Because a large number of spectra are recorded, and because eachspectrum is recorded during a time which is so short that the conditionsof measurement are constant, very small absorptions can be detected.

A number of embodiments of the invention will be described below,reference being had to the accompanying drawings. FIG. 1 illustratesschematically an apparatus according to the invention. FIG. 2illustrates schematically a variant of the apparatus shown in FIG. 1.FIGS. 3A and 3B illustrate schematically, in perspective and from above,respectively, an apparatus for sequential scanning of a spectrum. FIG. 4illustrates schematically an apparatus for sweeping a spectrum across adetector. FIGS. 5A and 5B illustrate absorption spectra of one and thesame substances, recorded at different temperatures. FIG. 6 is anabsorption profile and illustrates the profile broadening depending upontemperature and pressure.

FIG. 1 illustrates an apparatus adapted for measuring the parameters ofgases present in combustion processes and other high temperatureprocesses. A lamp 1 which must have at least the same frequency range asthe wavelength range one wishes to investigate, which must be continousand as stable as possible, and which may be, for example, a 450 wattxenon lamp, is positioned in the focus of a parabolic mirror 4 andadjacent the object 2 which is to be measured. Opposite the lamp 1 andon the far side of the object 2, which here is a flame, a receivingdevice comprising a parabolic mirror 5 and an oblique mirror 3 isprovided. If measuring is effected on light reflected or emitted by theobject of measurement, the receiving device may be positioned elsewherein relation to the lamp. A spectrometer 6 for spectral division of thelight is positioned such that it receives the light from the receivingdevice. Alternatively, the light from the object of measurement may beconducted directly into the spectrometer 6, in which case the receivingdevice will be superfluous. For measurements within the visible regionand within the IR and UV regions, the spectrometer suitably is aconventional grating spectrometer. The apparatus further comprises means7, 8 for sequential recording of the light spectrally divided by thespectrometer 6. In this embodiment, the said means include a rotarymirror 7 sweeping the spectrum across a fixed output slit from thespectrometer, as well as a light detector 8 which is disposed behindsaid output slit and preferably is a photomultiplier, for conversion ofthe intensity of the light transmitted through said slit, intoelectrical signals. In this instance, the spectrum thus is swept acrossa fixed slit, but it will be appreciated by those skilled in the artthat the slit may just as well be swept across a stationary spectrum. Inboth cases, scanning is carried out such that the location of the slitin the space relative to the spectrum is changed by mechanical means.FIGS. 3 and 4 below describe examples of suitable recording means. Afurther condition that must be fulfilled by said recording means 7, 8 isthat the spectrum can be recorded so quickly that the total lightintensity of the entire wavelength range will be constant during eachrecording. On the other hand, the total light intensity may be differentat different recordings because the signal level fluctuates independence on any turbulence in the object of measurement, vibrationsetc. The output of the apparatus is connected to a high-speed A/Dconverter, converting the analog signal from the photomultiplier intodigital format, whereupon the signal is stored in a computer 9. Storedin the computer 9 are programs for generating means values of therecorded spectra recorded under known conditions. and for controllingthe rotary mirror 7 as well as reference spectra recorded under knownconditions. The computer also has a memory space for storing therecorded spectra. A printer 10 or some other suitable display unit maybe connected to the computer. Furthermore, the computer 9 may beconnected to control means (not shown) receiving control signals inresponse to the measuring result.

The measuring apparatus operates as follows. Light from the lamp 1 isreflected in the parabolic mirror 4 which it leaves in the form of aparallel luminous beam which is transmitted through the object 2 to bemeasured. The light transmitted is received by the parabolic mirror 5,is reflected to the oblique mirror 3 and on to the input of thespectrometer 6. In the spectrometer 6, the light is spectrally divided.The rotary mirror 7 sweeps the spectrum across the fixed slit on thespectrometer output, and the photomultiplier 8 sequentially receives thelight coming from the different wavelength ranges of the spectrum andtransmitted through the slit, and provides an analog signalcorresponding to the intensity of this light. The analog signal is A/Dconverted and stored in the computer 9. By repeating these operations, alarge number of spectra (10,000-100,000) are recorded in a short time,whereupon the mean value of these spectra is calculated in the computer.To compensate for wavelength-depending variations in the output signalof the lamp and the reflection in mirrors etc., and to produce theinteresting absorption profiles, the mean value spectrum is divided by asuitable function. On the basis of the spectrum thus obtained, thecomputer calculates the required parameters in real time, as will beexplained below.

FIG. 2 illustrates a variant of the installation in FIG. 1. According tothis variant, the light is conducted from the light source 1 to theobject 2 to be measured and/or from said object 2 to the spectrometer 6by means of optical fibres 11. The light is introduced into the opticalfibres by means of a special device 12 comprising a planar obliquemirror and a focusing mirror (patent applied for, SE No. 8406025-0).This measuring installation is intended primarily for measurements inenvironments difficult of access, and for measuring across short andwell-defined distances, for example inside a flame.

FIGS. 3A and 3B illustrate an example of an apparatus for sequentialrecording of a spectrum. The apparatus comprises a rotary disk 30 whichis provided along its periphery with an upstanding ledge 30A with slits31 which are parallel to the axis of rotation of the disk 30 and whichare equidistantly spaced apart. Furthermore, a focusing grating 32 ismounted in the centre of the disk 30, and a photomultiplier 33 ismounted behind the ledge 30A.

Upon use of the apparatus, the disk 30 is caused to rotate by means of amotor (not shown), and the light is caused to impinge on a point abovethe ledge 30A through a fixed slit (not shown) located at the point ofintersection of the light rays, and on to the focusing grating 32 whichspectrally divides the light and reflects it against the periphery. Inthis manner, one slit 31 at a time will move through the spectrum, andthe photomultiplier 33 will sequentially receive and record light of thedifferent wavelengths of the spectrum. The advantage of this arrangementis that the slits 31 on the ledge 30A all the time will lie in the focalplane of the focusing grating 32.

FIG. 4 illustrates another apparatus for the sequential recording of aspectrum, said apparatus comprising a rotary mirror 36 located withinthe spectrometer 6 and receiving the spectrally divided light. Thenormal to the rotary mirror 36 deviates slightly from the axis ofrotation of the mirror. When the mirror is rotated by means of a motor(not shown), the incident light is reflected in an elliptical path onthe output slit 34 of the spectrometer 6, behind which slit aphotomultiplier 35 is provided for sequentially receiving light from thedifferent wavelengths of the spectrum. Furthermore, the apparatuscomprises a pin 37 provided at the periphery of the rotary mirror 36 andextending radially from said mirror, as well as a light barrier 38comprising an optical transmitter 39 and receiver 40. The light from thetransmitter 39 is stopped when the pin 37 passes, whereby a triggersignal for the measuring is generated.

The calculations made in the computer are based on the fact thatelectrons can move only in specific shells or orbits in atoms. Eachelectron orbit corresponds to a specific energy state. The situation israther more complicated in molecules. Besides the electron statesoccurring in the atoms, there exist also vibrational and rotationalstates which are due to the fact that the molecules can vibrate alongand rotate about, respectively, an axis. The principle, however, is thesame: each molecule has a limited number of permissible energy states.If an atom or a molecule is struck by photons, i.e. by light whosefrequency exactly corresponds to the energy difference between twostates in the atom or molecule, a photon is absorbed with someprobability, in which case the atom or molecule passes from one energystate to another. By transmitting light of a specfic frequency contentand a specific intensity through, for example, a gas, and by studyinghow much light has been absorbed at a specific frequency duringtransmission through the gas, much information is obtainable about thesubstances contained in the gas.

A review of how concentration, temperature, pressure, particle sizedistribution and number of particles are calculated on the basis of therecorded spectra, will be given below.

TEMPERATURE

The electrons in an atom occupy different states depending upon thetemperature. In the same manner, different electron, vibrational androtational states in a molecule are occupied in dependence upon thetemperature. This means that atoms and molecules absorb photons ofdifferent frequencies in dependence upon the temperature, and this inturn results in different appearances of the absorption spectrum of asubstance at different temperatures. FIGS. 5A and 5B illustrate twodifferent spectra for sulphur dioxide, recorded at differenttemperatures in a 100 MWatt power plant straight through the flame. Thedifferences are clearly apparent. By comparing a spectrum recorded bythe apparatus according to the present invention, with spectra recordedduring (or calculated for) known conditions, the temperature can bedetermined.

CONCENTRATION

The concentration is determined by means of the Lambert-Beer law I=I₀e⁻ρlc wherein I represents the intensity of the transmitted light, I₀represents the intensity of the light source, ρ represents theabsorption cross-section of the substance in question, 1 represents theabsorption distance, and c represents the concentration. For thisdetermination of the concentration, however, the temperature must beknown since the absorption cross-section is temperature dependent.However, the temperature can be readily determined by the methodaccording to the present invention. The concentration thus determined isthe mean concentration of the substance in question along the absorptiondistance. If the temperature is different at different points along theabsorption distance, further information may be obtained.

PRESSURE

FIG. 6 illustrates an absorption profile, especially the broadening ofthe profile due to pressure and temperature. Curve A shows the truetemperature contribution, so-called Doppler broadening. The half-widthvalue of this curve is proportional to the root from the temperature.Curve B shows the appearance of the absorption profile upon pressurebroadening. The half-width value of this curve is directly proportionalto the pressure and proportional to the root from the temperature. Thesecurves thus show the broadening phenomena separately. The actualabsorption profile will be a combination of the two. If the temperatureis known, one can thus determine the pressure as a function of thebroadening of the absorption profile and vice versa.

NUMBER OF PARTICLES AND THEIR SIZE DISTRIBUTION

When the number of particles in a gas and the particle size distributionare to be determined, a spectrum of the entire optical region from UV toIR is recorded. The so-called Mie effect causes light of differentwavelengths to be scattered differently against particles in the gas,and thus implies that different amounts of light in different wavelengthregions will reach the receiving device. By comparing the intensity ofthe transmitted light in different wavelength regions in the spectrum,the particle size distribution can be determined, and by studying thelevel of the intensity of the entire wavelength region, the number ofparticles can be determined.

To be able to make the above-mentioned calculations it is a conditionthat a spectrum of sufficient accuracy can be recorded. This is madepossible by the rapid sweeping and recording of the spectrum on thespectrometer output by means of a one-channel detector, and the greaterthe number of recordings, the higher the accuracy. More particularly,the noise decreases with the root from the number of recordingseffected.

This technique has many advantages. It is far cheaper than thepreviously described laser-based method. Furthermore, a very highmeasuring accuracy can be obtained when measuring is effected onturbulent objects, by using a continuous light source and a one-channeldetector, as well as by very rapid scanning of the spectrum. In manycases, the accuracy is better than can be achieved by other measuringtechniques. It is a generic technique. The same apparatus may be usedfor measurements on different substances and for different applications.The measuring technique also is simple and reliable, and no skilledtechnicians are required for operating the equipment. Besides, thetechnique is suitable for long-term measurements since it need not beconstantly supervised. It also is suitable for measurements at locationsdifficult of access since the light can be conducted to and from thepoint of measurement by means of optical fibres, which is not possiblewith the laser light of prior art technique. In addition, one and thesame measuring apparatus can be used for monitoring several processes orseveral points of measurement in one process by conducting light fromone or more lamps to the different points of measurement, and from thedifferent points of measurement to the measuring apparatus proper bymeans of optical fibres, the apparatus being controlled to cyclicallycalculate the parameters of the different measurements. If severalabsorption distances are recorded simultaneously in different directionsthrough the object of measurement, three-dimensional maps of therequired measuring values can be generated by tomography.

The present invention is intended for real-time monitoring of andmeasuring in combustion processes, especially flames, and in allprocesses where high temperatures prevent the use of conventionaltechnique. The present invention also is intended to function as atransucer for controlling processes of the above-mentioned type.Examples of applications are the heat-generating and power industries(combustion of various fuels, flame monitoring), chemical processindustries (temperature monitoring in hydrochloric acid furnaces), thepaper and pulp industry (determination of particle content, detection ofgases at high temperatures), the iron and steel industry (temperaturemeasurement in furnaces and converters, analysis of heavy elements ingas flows), the automobile industry (exhaust gas analysis, especially inconnection with catalytic exhaust gas purification), etc.

The present invention naturally is not restricted to the embodimentsillustrated, and many modifications can be made within the scope of theappended claims. For example, the technique according to the presentinvention has been described with reference to an embodiment in which alamp is used as the light source and an absorption spectrum is recordedby means of the light transmitted through the object of measurement. Itis, however, also possible to record absorption spectra by means of thereflected light. Furthermore, the actual object of measurement may beused as a light source, and a spectrum is recorded of the emitted light.

We claim:
 1. A method for determining a parameter such as pressure,temperature or concentration of a gaseous substances present incombustion processes and other high temperature processes, comprising(a)transmitting spectral broad-band, continuous light towards the gaseoussubstances, a part of the light being absorbed by the gaseous substancesand a part of the light being transmitted through said gaseoussubstance; (b) spectrally dividing the transmitted light, whereby theabsorption spectrum of the gaseous substances is obtained; (c) recordingthe absorption spectrum of the gaseous substances in a studiedwavelength range a large number of times, each recording taking placesequentially by sweeping the absorption spectrum relative to asingle-channel detector and for such a short time that the totalintensity of the entire wavelength range is constant during eachrecording; (d) generating the means value of the recorded absorptionspectra; and (e) calculating the parameter on the basis of theappearance of the mean absorption spectrum and absorption spectradetermined for known conditions.
 2. An apparatus for determining aparameter such as pressure, temperature or concentration of a gaseoussubstances present in combustion processes and other high temperatureprocesses, comprising(a) a spectral broad-band, continuous light sourceadapted to transmit light towards the gaseous substances; (b) means forspectrally dividing the light transmitted from said light source andthrough said gaseous substances, to obtain the absorption spectrum ofsaid gaseous substances; (c) recording means for sequentially recordinga large number of times, the absorption spectrum of said gaseoussubstances in a studied wavelength range, said recording meanscomprising a one-channel detector whose output signal is proportional tothe intensity of the received light and means for sweeping theabsorption spectrum relative to said one-channel detector, and saidrecording means being adapted to perform each recording in such a shorttime that the total light intensity of the studied wavelength range isconstant; (d) a mean value generator for generating the mean value ofthe spectra recorded by said recording means; and (e) calculating meansfor calculating the parameter.
 3. An apparatus as claimed in claim 2,wherein said means for sweeping the absorption spectrum in relation tosaid one-channel detector is a rotary disk which is provided along itsperiphery with slits extending parallel to the axis of rotation of saiddisk, and which carries in its center a means for spectral division ofthe light and focusing the light against said slits.